This invention relates to martensitic stainless steels used in seamless steel pipe such as oil-well pipe and pipeline tubing. Martensitic stainless steels, two representative grades of which are SUS 410 and SUS 420 (Japan Industrial Standard JIS! designations), have excellent corrosion resistance in highly corrosive environments containing CO.sub.2. These materials are thus regarded as excellent candidates for use in oil-well pipe, geothermal well pipe, and pipeline tubing.
The strength of oil-well pipe is normally required to be least equivalent to that of American Petroleum Institute (API) standard L80 grade steel (yield strength.gtoreq.80 ksi). Pipeline tubing should Generally have a strenoth at least equal to that of API standard X60 grade steel (yield strength.gtoreq.60 ksi).
Martensitic stainless steels having a variety of strengths can be obtained by the application of specific types of heat treatment, such as quench-tempering, normalizing-tempering, or just tempering. However, it is known that the resistance to stress-corrosion cracking of martensitic stainless steels in CO.sub.2 -containing environments falls when tempering is performed at a temperature of less than 600.degree. C. We tempered the martensitic stainless steels in Table 1 at various temperatures and cut out test pieces 120 mm long by 20 mm wide by 3 mm thick. These pieces were subjected to U-bend tests at a bending radius of 15 mm in a 3.5% aqueous solution of NaCl heated to 80.degree. C. and having a carbon dioxide partial pressure of 1 atmosphere. As shown in Table 1, each of these steels developed stress-corrosion cracking when tempered at less than 600.degree. C., but none demonstrated stress-corrosion cracking when tempered at 600.degree. C. or more (a cross "X" in Table 2 indicates the presence of stress-corrosion cracking; an open circle ".largecircle." indicates the absence of stress-corrosion cracking). Hence, to achieve good resistance to stress-corrosion cracking in CO.sub.2 -containing environments, an important property of martensitic stainless steels, it is clear that the steel must be tempered at a temperature of at least 600.degree. C.
TABLE 1 ______________________________________ Tempering temperature (.degree. C.) Type of steel 450 500 550 600 650 700 750 ______________________________________ 0.25C. - 13Cr X X X .largecircle. .largecircle. .largecircle. .largecircle. 0.2C. - 13Cr X X X .largecircle. .largecircle. .largecircle. .largecircle. 0.15C. - 13Cr X X X .largecircle. .largecircle. .largecircle. .largecircle. 0.10C. - 13Cr X X X .largecircle. .largecircle. .largecircle. .largecircle. 0.03C. - 13Cr - 3Ni - X X X .largecircle. .largecircle. .largecircle. 0.5Mo 0.03C. - 13Cr - 2.5Ni - X X X .largecircle. .largecircle. .largecircle. .largecircle. 1Mo 0.005C. - 13Cr - 3Ni - X X X .largecircle. .largecircle. .largecircle. .largecircle. 0.5Mo ______________________________________
As is well known, the strength of martensitic stainless steels decreases as the ferrite content of the steel structure increases. When the ferrite content at 1200.degree. C. exceeds 40%, the ferrite content in the normal quenching or normalizing temperature range of 900-1000.degree. C. rises to 20% or more, making it difficult to achieve the high strength required in linepipe tubing and oil-well pipe by tempering at 600.degree. C. or more. Accordingly, to allow tempering to be performed at the temperatures of 600.degree. C. or more necessary to impart good resistance to stress-corrosion cracking, and at the same time satisfy the high-strength requirements for use in pipeline tubing and oil-well pipe, martensitic stainless steels must be composed of not more than 40% ferrite at 1200.degree. C.
Compositions in which the austenite phase (which becomes martensite at room temperature) exists in combination with a ferrite phase comprising 20-30% of the composition have the worst hot workability. When the amount of ferrite is about 40%, the hot workability is about the same as that of austenitic single-phase steels (which become martensitic single-phase steels at room temperature or below the Ms point). The hot workability rises sharply with increasing ferrite content above this point. Thus, because martensitic stainless steels with a ferrite content of 40% or less at 1200.degree. C. have inferior hot workability, their use in the production of high-strength seamless steel pipe by the processes described below tends to result in defects, complicating pipe manufacture.
Seamless stainless steel pipe is generally produced either by an inclined rolling method such as the plug mill or mandrel mill process, or by a hot extrusion method, of which the Ugine-Sejournet and Erhart pushbench processes are typical. However, certain types of martensitic stainless steels (namely, those with a ferrite content of 40% or less at 1200.degree. C.), have poor hot workability. When seamless steel pipe is manufactured from these steels by a cross rolling process such as the plug mill process or the mandrel mill process, defects arise on both the outside and inside walls of the pipe during piercing of the billet on a piercing mill. For this reason, seamless pipe made of this type of steel is generally produced by a hot extrusion process, such as the Ugine-Sejournet process.
However, when a hot extrusion process is employed and the billet directly pierced (a process known as direct piercing), a billet length 5-7 times the diameter results in a greater eccentricity in the wall thickness of the pipe. This makes it difficult to produce long pipe. A partial solution to this problem is provided by a process for producing long pipe that makes use of what is known as the expansion method. This method consists of mechanically opening a guide hole in the center of the billet, then extending the hole. However, even with the use of this expansion method the billet length is still limited to only about 15 billet diameters. Another problem concerns the glass lubricant used in the Ugine-Sejournet hot extrusion process. This must be peeled off following rolling, a process that is both time-consuming and costly.
The limits on billet length inherent in the Ugine-Sejournet and other hot extrusion processes make it impossible to raise productivity above a certain level. Moreover, the use of short billets inevitably results in a low yield and is therefore also disadvantageous in terms of cost. In contrast, both the plug mill and mandrel mill processes involve piercing the billet on a piercing mill that utilizes the Mannesmann effect. These processes permit the manufacture of longer pipe than is possible by the Ugine-Sejournet and other hot extrusion processes. These processes are thus known to be advantageous in terms of productivity and cost. However, as indicated above, certain types of martensitic stainless steels are not suitable for use in the production of seamless pipe on account of the formation of defects during pipe manufacture.
The present invention was arrived at following careful consideration of the problems described above. The object of this invention is to enable the practical application of the plug mill and mandrel mill processes in martensitic stainless steels, particularly those having a ferrite content of 40% or less at 1200.degree. C., for which the manufacture of seamless steel pipe by the plug mill and mandrel mill processes has hitherto been complicated by the formation of defects during pipe fabrication, and by making it possible to use these processes, to enable the manufacture of seamless steel pipe from this type of martensitic stainless steel at high productivity and low cost.
To recapitulate, it has hitherto been possible to manufacture seamless steel pipe made of martensitic stainless steel by the plug mill process or the mandrel mill process without the formation of defects during fabrication when the ferrite content is greater than 40% at 1200.degree. C. However, when steels having a ferrite content of 40% or less at 1200.degree. C. are used, numerous defects arise on the inside and outside walls of the pipe during manufacture, and cracking at the ends of the pipe is also common. This has made it difficult to use these processes in the production of seamless steel pipe. After carefully investigating the causes of such defects in steels with a ferrite content of 40% or less, we discovered that impurities such as P and S in the steel exert a large influence on the formation of these defects. Further examination revealed that by holding the level of S to 0.003% or less and the level of P to 0.020 or less, seamless steel pipe can be produced on a practical basis by both the plug mill process and the mandrel mill process. This discovery led ultimately to the present invention.